Numeric don't compare arrays correctly

Alexander Schmolck a.schmolck at gmx.net
Fri Jun 6 22:45:36 CEST 2003


Alexander Schmolck <a.schmolck at gmx.net> writes:

> Maurix <maurix78_remove_this_ at wanadoo.es> writes:
> 
> > Hi to all,
> > I'm new in python world and in this group too so, nice to meet you all ;-)
> > I have a problem with Numeric: (linux, python2.2, numeric 21.0)
> > 
> > It seem that don't compare correctly arrays.
> > 
> > See at this session:
> > 
> >  >>> a=array([1.1,2.2,3.3,4.4,5.5])
> >  >>> print 2.<a<4.
> 
> This is just a python wart, reflecting the unhappy interaction between two
> sometimes convinient but somewhat messy language features.
> 
> Feature 1:
> 
> a < b < c in most programming languages is the same as (a < b) < c. Python
> instead treats such chained comparisons specially to make them behave as is
> typical in (the somewhat messy) standard math notation.
> 
> So ``1 < 3 < 4`` is False and not True, because it is treated as 
> ``1 < 3 and 3 < 4``. 

Damn, yet another self-correction: This example should read 

``3 < 1 < 4`` (False in python, True in most other languages where it isn't a
syntax error)


> 
> Feature 2:
> 
> Rich comparisons. a > b can return *anything*, not just 0/False 1/True. In
> particular it is used. In particular, Numeric returns elementwise comparison
> arrays for ``array1 < array2``. My personal feeling is that allowing this was
> a bad idea one symptom is that it clashes with Feature 1 (
> ``array1 < array2 < array3`` = ``array1 < array2 and array 2< array 3`` =
> ``array` < array 2`` if at least one element in array1 is smaller than in
> array 2 = not what you most likely want).
> 
> 
> 'as

'as




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